The most basic type of drilling rig includes a joint or string of joints of drill rod having a bit mounted at the bottom end thereof. The drilling rod is mounted to a ram which applies down pressure to the drill rod string as it rotates to effect the drilling function. As one joint of drill rod becomes completely submerged beneath the level of the ground, an additional joint can be attached to the upper end of the previous joint and fixedly attached to the ram to complete the rig.
A more sophisticated type of rig is characterized as a percussion rig. In such a rig, an assembly which can include a rotatable bit extends downwardly into the hole being dug, by one or a series of connected drill rod joints. The assembly further includes a percussion piston which is caused to impact upon an upper end of the drill bit element. In most rigs of this type, the bit element includes an annular lip which axially engages a retaining ring detachably mounted within the assembly to maintain the bit suspended from the assembly. The cooperation of the structure of the bit element and the rest of the assembly is such that the bit may slide within the assembly axially with respect to the hole being drilled.
The percussion piston is pneumatically activated typically by high pressure air which is made to flow from the rig downward through the joints of drill rod. This activation is effected when the bit element is engaging the bottom of the hole and the constant down pressure of the ram is being applied to the percussion assembly. The annular lip formed in the bit element is, thereby forced to withdraw axially upward away from the retaining ring. With the bit element in this relationship with respect to the rest of the percussion assembly, the activation fluid is directed to cause impacting of the percussion piston upon the upper end of the drill bit element.
In rigs of this type, both rotational motion and vibratory motion parallel to the direction of the drilled hole can be imparted to the bit simultaneously. Such a rig is particularly useful when the ground through which the hole must be drilled is unusually hard or rocky.
The pneumatic fluid which can be used to activate the percussion piston can also serve an additional function. The downward pressure applied by the ram can be abated and the string of drill rod withdrawn from the hole slightly so that the annular lip formed in the bit element engages the retaining ring with the bit element, thereby, suspended from the ring. With the bit element so disposed with respect to the rest of the percussion assembly, the HP air can be made to exit through ports formed in the bit proximate the interface between the cutting surface of the bit and the ground. The fluid can thus serve to entrain loose drilled matter, or cuttings, within an air flow created upwardly within the hole and blow these cuttings out of the hole.
In certain circumstances, percussion drilling is used in ground composed of soft material which can fall from the sides of the drilled hole as the drill rod is withdrawn therefrom. The hole can, therefore, become filled with loose materials so that the effect of the drilling is, to some degree, negated. These problems can be overcome by simultaneously inserting a casing within the hole around the drill string to support the inner surface of the hole. With the casing serving as an inner retaining wall, when the drill rod and bit assembly are withdrawn, cuttings will not fill the hole.
When drilling and casing insertion functions are effected simultaneously, since the inside diameter of the case must, necessarily, be larger than the outside diameter of the bit in order that the bit can be withdrawn, pressure must be applied to the casing at its end external to the hole in order to drive it down around the drill rod. There are structures currently in use today which can simultaneously accomplish the drilling and casing driving functions. In such devices, however, separate assemblies are used to effect each of the functions. Consequently, such devices tend to be bulkier than more conventional drilling rigs.
Additionally, these dual function devices have tended to make the older, more conventional rigs obsolete in a sense, at least as applicable to dual function drilling. Drilling companies have, therefore, been required to purchase the newer devices whenever contracting to do a drilling job which requires a rig capable of accomplishing the dual functions.
It is these problems in the art which the invention of the present application is designed to overcome. The invention provides a structure which facilitates conversion of the older, conventional, bottom-hole drilling rigs into rigs which can serve the dual functions of operating the drill bit and driving the casing down around the drill rod.